This invention relates to thin film gas-barriers. More particularly, it relates to methods for characterizing thin film gas-barriers using gas-phase synthesis of quantum dots.
In the electronics industry, much work has been carried out into the development of effective gas-barriers for electronic devices. Gases such as O2 and water vapor can detrimentally affect the stability and performance of electronic devices. Therefore a gas-barrier film may be applied on top of the active device layer to shield the active layer from such contaminants. In addition, gas-barrier films may impart functionality, such as flexibility or shock-resistance, to electronic devices.
Gas-barrier films are employed in other areas in addition to the electronics industry. For example, in the packaging industry gas-barrier films are used to protect foods and pharmaceuticals from bacteria and other contaminants.
Typical gas-barriers are organic, inorganic, or inorganic-organic hybrid materials. Examples of organic materials include polymers (acrylates, epoxides, polyamides, polyimides, polyesters, cellulose derivatives, etc.), which are often hydrophobic in nature. Inorganic gas-barriers may include dielectric (insulating) materials, metal oxides, metal nitrides, or silica-based (glass) materials. For inorganic-organic hybrids, the organic component is often a polymer, e.g. a silica-acrylate hybrid.
For a gas-barrier film to function effectively, it must be impenetrable to small molecules. Therefore, any cracks or channels in the film must be eliminated. Some examples of potential defects in a thin film are illustrated in FIG. 1. Such defects in a film 100 include open pores 101, pinholes/closed pores 102 and cracks 103. Closed pores are inaccessible to small molecules and therefore do not compromise the film integrity. However, such pores are still undesirable as they may weaken the film. Full-thickness cracks compromise the film integrity by allowing gas molecules to diffuse through, potentially reducing the device stability and performance. Defects in contact with just one surface of a film are also undesirable since they have the potential to propagate to full-thickness cracks.
As such, it is necessary to have a non-destructive method to detect defects in gas-barrier films. It is also advantageous to have a testing method that tests the gas-barrier's penetrability to gases. In addition, a detection technique that may characterize the defect structure of a film, rather than just defect pore size distribution, is highly desirable.
Methods of the prior art used to detect defects in films include mercury intrusion porosimetry, and nitrogen gas adsorption. Mercury intrusion porosimetry is a method used to characterize pores in solid films, particularly in the packaging in the pharmaceutical industry. Using low-pressure mercury, pore diameters between 14-200 μm may be detected, while high pressure mercury porosimetry may be used to detect pore diameters down to 3 nm. The technique was first developed by Ritter and Drake in the 1940s [H. L. Ritter & L. C. Drake, Ind. Eng. Chem. Anal. Ed., 1945, 17, 782]. The technique exploits the dependence of the rate of penetration of a liquid into a medium on the size and distribution of pores within the medium. Elemental mercury is ideally suited to this technique since it is non-wetting to most solids. The pressure required to force a non-wetting liquid into a capillary of circular cross-sectional area is inversely proportional to the capillary diameter and is directly proportional to both the surface tension of the liquid and the contact angle made with the solid surface. Therefore, for a given applied pressure it is possible to calculate the size of the pores that Hg will enter and those it will not.
However, the 3 nm lower detection limit of mercury intrusion porosimetry is inadequate for verification of the integrity of gas-barrier films, since molecules of water and oxygen are an order of magnitude smaller and may diffuse through much narrower cracks. Further, mercury intrusion porosimetry may be used to give an indication of defect diameter, but does not provide any visual representation of the defect structure. The model assumes a cylindrical pore shape, which is not always accurate. Using conventional apparatus, the detection area is also limited to approximately 1 cm2, so larger films cannot be characterized by this technique. The technique relies upon high operating pressures, which may be damaging to the film or electronic device. These high pressures may be particularly detrimental when a sample contains many closed pores, which cannot be detected using this technique, as the high pressures may compress a sample. Mercury intrusion porosimetry has also been found to overestimate the diameter of very small pores.
Gas adsorption porosimitry may be used to determine the pore size distribution of a material from its gas adsorption isotherm [C. G. Shull, J. Am. Chem. Soc., 1948, 70, 1405]. Nitrogen gas adsorption is used in the detection of pores with a wide range of diameters from 3 Å to 300 nm. The technique may be used to determine pore volumes, along with the volume pore size distribution, based on a similar technique to mercury intrusion porosimetry, except replacing the non-wetting liquid with nitrogen gas. In the pore size range where capillary condensation occurs, capillary condensation into pores for a condensate with a concave meniscus will ensue provided the pressure of the adsorbate exceeds the equilibrium pressure of liquid in the pore. The pore size distribution may be determined from the adsorption or desorption isotherm.
As with mercury adsorption porosimetry, the nitrogen gas adsorption technique assumes a cylindrical pore shape, with open pores, and an absence of pore networks. Therefore, it may be inaccurate in characterizing the pore distribution in systems wherein the pore structure deviates from these criteria. In addition, the acquisition of a single measurement may take several hours. Other drawbacks include the fact that the sample may come into contact with liquid nitrogen during the course of the measurement, which may result in damage to the sample, and cryogenic temperatures are required to ensure little attraction between the gas-phase molecules, which may be difficult and costly to maintain.